My mother took my sisters 38 special to them as my sister had moved to Germany and the firearm license was expiring

My mother took my sisters 38 special to them as my sister had moved to Germany and the firearm license was expiring. They offered to buy it for R1000. They sold the firearm and had it registered in the new owners name but still haven't paid the money offered over to my sister. We have tried on more than one occasion to get them to pay the money and no joy. We have phoned and we have sent emails.